This invention relates to fractional distillation apparatus, and more particularly, to fractionating columns of the spinning band type.
In the chemical industry, it is common practice to use distillation apparatus in order to separate liquid mixtures having different boiling points. Although various types of stills have been used, most have common features: a still pot holding a charge to be separated, a vertically disposed fractionating column immediately above and communicating with the still pot, a condenser section located at the upper portion of the column, controllable heater means disposed in the still pot, and liquid take-off means running from the condenser. In order to assure an adiabatic separation process, many stills incorporate a vacuum jacket, often silvered, in order to prevent heat transfer around the fractionating column.
Efficiency in a fractionating still is a measure of the degree of enrichment or rectification of one compound as it is being separated from others within the column. This separation is invariably achieved through the intimate contact between the vapors rising from the still pot and the liquid or reflux descending within the column. As this liquid/vapor contact occurs, the result is that only the most volatile material proceeds upward towards the receiver, while the less volatile material returns downward as a liquid towards the still pot. Thus, the efficiency of the separation process depends on the frequency and degree of contact between the ascending vapor and the descending liquid. As this contact becomes more intimate and frequent, the fractionation will become more efficient.
As a separating medium, a number of stills of the `packed column` type have employed various types of stationary fillers situated in the column in order to result in effective liquid/vapor contact. These packings have included glass beads, ceramic saddles, glass and metal helices, metal screens formed into cones, metal chains, rings, conical discs, and a number of other materials of different shapes, all fitted very tightly between the walls of the column. Although these packed column stills can provide a high degree of fractionation, they do have a number of shortcomings. One is that effective operation is conditional on proper packing, and that packings often become disarranged in the course of distillations. Sometimes, the amount of packing is so great that the holdup inherent in these types of stills renders them unsuitable for fractionating small quantities of liquids. When operating at reduced pressure, the packed column design makes it extremely difficult to obtain both an acceptable separation efficiency and a relatively low pressure drop between the still and the column head, the latter being especially crucial in avoiding decomposition of compounds having high molecular weights.
In addition to distillation columns having stationary packings, a number of separating columns having mechanically moving parts have been proposed. These types of columns usually spin some type of band, cone, brush, or wiper in the center of the column in order to affect better mixing and contact of the falling liquid with the rising gas. In general, these types of columns have been extremely successful at affording good overall liquid/vapor dispersion (i.e., high efficiency), very low holdup in the column, high numbers of theoretical plates for a given column length, and low pressure drop as compared to their packed column counterparts.
Yet, in the early development of these types of columns involving moving parts, problems associated with fractionation at high vacua often arose. Ordinary packings and bearings were not reliable in maintaining sufficiently tight seals around the rotating members. Fortunately, the development of magnetic as well as other specialized types of bearings has eliminated all vulnerable leakage points around the moving parts of these stills, and they are now extremely reliable when run at reduced pressure.
One specific type of distillation apparatus which spins a band throughout a major portion of the length of the column is called the `spinning band column`. Generally, the spinning bands incorporated in these types of distilling columns take on a spirally wound shape, and are just wide enough to lightly scrape the surrounding walls of the column when they are rotated. The spiral shape, giving the band the appearance of an elongated helicoid, produces an axial thrust as the band is spun at high rpm's. Thus, when rotated at high speeds and in the proper direction, these spinning bands force the reflux downward along the walls of the column in a quick and uniform manner. This latter feature is especially helpful in preventing these types of fractionating columns from flooding, even when operated at high boil-up rates.
Many efforts regarding spinning band columns have focused on improvements to the spinning band itself. As previously mentioned, efficiency is dependent on the liquid/vapor contact within the column. Towards this end, most advances in these types of columns have come from improvements to the spinning band which facilitate better mixing of the liquid and gas within the column. It is with this objective that the present invention is concerned.